Every kind of water is freshest in winter, not so fresh in summer, still less so in autumn, and least of all in times of drought. River-water, too, is by no means always the same in taste, the state of the bed over which it runs making a considerable difference. For the quality of water, in fact, depends upon the nature of the soil through which it flows, and the juices[1] of the vegetation watered by it; hence it is that the water of the same river is found in some spots to be comparatively unwholesome. The confluents, too, of rivers, are apt to change the flavour of the water, impregnating the stream in which they are lost and absorbed; as in the case of the Borysthenes, for example. In some instances, again, the taste of river-water is changed by the fall of heavy rains. It has happened three times in the Bosporus that there has been a fall of salt rain, a phænomenon which proved fatal to the crops. On three occasions, also, the rains have imparted a bitterness to the overflowing streams of the Nilus, which was productive of great pestilence throughout Egypt.